1. Field of the Invention
The present invention concerns a liquid crystal display, and relates especially to a normally black twisted nematic liquid crystal display with a nematic liquid crystal material interposed between the front electrode and the rear electrode.
2. Description of the Prior Art
A normally black twisted nematic liquid crystal display is normally composed of a front electrode, which has been subjected to an orientation treatment in an obliquely upward direction (typically, 45.degree. in the upper right direction or in the upper left direction for the given typical example); a rear electrode, which has been subjected to an orientation treatment in an obliquely downward direction (typically, 45.degree. in the lower right direction or in lower left direction for the given typical example); a nematic liquid crystal material laid between the two electrodes to be twist-oriented by 90.degree., and front and rear polarizing plates provided in front of and behind the front and rear electrodes respectively in a direction coinciding with the orientation of either the front electrode or the rear electrode. Thus when no voltage is applied between the front and rear electrodes, the polarizational direction of light deviates from the polarizational axis of the front polarizing plate by the 90.degree. twist orientation of the liquid crystal material, so that no light can pass ahead of the front polarizing plate; on the other hand, when a voltage not less than a definite threshold value is applied between the front and rear electrodes, the twist of the liquid crystal material is released to pass light.
In such a liquid crystal display, conventionally the optical path length difference, .DELTA.nd, of the liquid crystal material (the product of the thickness of the liquid crystal material between the front and rear electrodes, that is, cell thickness d, and the double-refraction index, .DELTA.n, of the liquid crystal material) is so set that the transmittance with no voltage applied between the electrodes can be minimized when the display panel is viewed in front.
Such a method of setting a .DELTA.nd enables a contrast ratio observed in front of the display panel to be maximized, yet on the other hand involves some problems, e.g.: the black level (background color) and the contrast ratio vary with visual angles; this method necessitates thinning the cell thickness and therefore the contrast and chromaticity might vary even with a slight manufacture error in cell thickness.
In order to solve these problems, the inventor proposed in Japanese Patent Application No. 63-283760 that the variations in contrast and black level (background color) with visual angle changing be reduced by limiting the optical path length difference to not smaller than 1.4 .mu.m and not longer than 1.7 .mu.m.
Even though the technique disclosed in the above-mentioned Patent Application No. 63-283760 may be employed, there still remains the problem that in vertically long liquid crystal displays, changes in contrast and black level (background color) tend to increase as the visual angle is changed upward or downward, since in the vertically long liquid crystal displays it is natural that their visual angles increase in the vertical direction.
The present invention is intended to solve the above-mentioned problem and to provide a liquid crystal display in which, when the upward or downward visual angle is varied, the variation in contrast ratio and black level (background color) is small, and, moreover, a sufficient contrast ratio can be obtained.